1. Field of the Invention
The present invention relates generally to a semiconductor process, and more specifically to a semiconductor process that forms an epitaxial structure after a source/drain is formed.
2. Description of the Prior Art
For decades, chip manufacturers have made metal-oxide-semiconductor (MOS) transistors faster by making them smaller. As the semiconductor processes advance to very deep sub micron era, such as 65-nm node or beyond, how to increase the driving current of MOS transistors has become a critical issue. In order to improve the device performances, crystal strain technology has been developed. Crystal strain technology is becoming more and more attractive as a mean for getting better performances in the field of CMOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through said crystal. Strain makes CMOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel.
In the known arts, attempts have been made to use a strained silicon layer, which has been grown epitaxially on a silicon substrate with a silicon germanium (SiGe) or a silicon carbide (SiC) layer disposed in between. In this type of MOS transistor, a biaxial tensile strain occurs in the epitaxy silicon layer due to the silicon germanium or the silicon carbide (SiC) which lattice constant is larger or smaller than the silicon one, and, as a result, the band structure is altered, and the carrier mobility increases. This enhances the speed performance of the MOS transistors.
However, the epitaxial layer such as the silicon germanium (SiGe) or silicon carbide (SiC) epitaxial layer will suffer thermal processes such as a source/drain annealing process or etc in semiconductor processes. These thermal processes cause dopants of the epitaxial layer to diffuse outwards, leading to electrons passing through the gate channels which provokes circuit leakage, and accordingly reduces the forcing stress effect of the epitaxial layer.